Present regulatory conditions in the automotive market have led to an increasing demand to reduce emissions in present vehicles. Catalytic converters and NOx traps or absorption units are among the primary tools used to reduce emissions in vehicles.
A three-way catalytic converter oxidizes hydrocarbons (HC) and carbon monoxide (CO) emissions in a vehicle into relatively benign compounds such as carbon dioxide (CO2) and water. A catalytic converter typically includes a specific catalyst formulation including platinum, palladium, and rhodium to reduce oxides of nitrogen (NOx), HC and CO simultaneously. The conversion efficiency of a catalyst depends on the temperature of the catalyst and the air/fuel ratio. NOx traps are used to capture NOx during lean operating conditions and release NOx under rich operating conditions, such that the NOx is reduced to carbon dioxide and nitrogen (N2)
The functional temperature range for a typical lean NOx trapping and three-way conversion catalyst is shown in FIG. 1. As shown in this example, peak NOx conversion efficiency is obtained in substantially the 250° C.–450° C. temperature range. These temperatures may vary somewhat based on the specific formulation of precious metals and NOx trapping chemicals. These temperatures are high enough to simultaneously clean up the HC and CO emissions. The catalyst formulation shown in FIG. 2 effectively traps NOx during lean air-fuel operation by catalyzing NO to NO2, and then chemically storing it as a nitrate (NO3) compound on the washcoat surface. When all the NOx storage sites are filled, a reducing (oxygen deficient, CO rich) exhaust environment must be created in the catalytic converter. The reducing environment causes the stored nitrate (NO3) to be released as gaseous NO2. The NO2 can be further reduced to nitrogen, N2, at a precious metal site, such as platinum, if sufficient reductants such as HC, CO and H2 are present.
The typical precious metal catalyst formulations maintain very high conversion efficiencies up to temperatures of 900° C. The NOx storage compounds, such as barium or potassium, that are added to the three-way catalysts are usually stable up to temperatures of approximately 850° C.
Lean NOx trap performance on diesel engine applications is severely limited by the lower exhaust gas temperatures and the difficulty of providing frequent, rich exhaust mixtures to the catalyst. (Lean and rich air-fuel ratios are described in the next paragraph) The diesel engine's very low exhaust temperatures are the result of very lean operation, and higher compression and expansion ratios. These are the same attributes that account for the higher fuel efficiency compared to the spark ignited gasoline engine.
Air-fuel ratios may also be adjusted to vary catalytic performance in a vehicle. An air-fuel mixture is represented by a ratio called the equivalence ratio that is represented by the symbol λ.
The equivalence ratio is defined by the following equation:
  λ  =            (              air        ⁢                  /                ⁢        fuel            )              (              air        ⁢                  /                ⁢        fuel        ⁢                                  ⁢        stoichiometry            )      
A relatively low air/fuel ratio below 14.7 (λ<1) is characterized as a rich mixture, and an air fuel ratio above 14.7 (λ>1) can be characterized as a lean mixture. Traditional vehicle gasoline engines are operated at stoichiometry (λ=1) since most regulated exhaust gases can be reduced simultaneously at stoichiometry. If vehicle engines are operated on lean mixtures, such as diesel engines, the NOx compounds generated may not be sufficiently reduced by traditional three-way catalysis. Therefore, these engines have difficulty meeting the increasingly stringent exhaust emissions regulations.
Present systems to improve NOx emissions for diesel-powered vehicles include urea selective catalytic reduction (SCR) and lean NOx trap systems. Urea SCR utilizes a urea supply ((NH2)2 CO) and exhaust dosing system to provide ammonia (NH3) to the feedstream of a large, specialized catalyst. The ammonia then reduces the NOx emissions to N2 over the selective catalyst. Urea SCR requires a separate urea tank in a vehicle and the development of a urea fueling infrastructure. The Lean NOx Trap system further requires a specially formulated three-way catalyst with added NOx storage chemistry. The NOx emissions that are chemically stored during lean operation must be released and reduced in a net, fuel-rich environment on the catalyst surface. The frequent generation of rich, reducing exhaust pulses poses significant challenges to diesel engine systems.